Nucleic acids play a key role in cellular functions. Because chemical alterations in active nucleic acids by UV-, x-, and gamma-ray radiation produce mutagenic, lethal and other biological effects, photochemistry, photobiology, radiation chemistry and radiobiology of nucleic acids have been in the foreground of scientific research for the past three decades. Therefore, in order to understand the action of ultrasound on biological systems, studies on ultrasonic radiation of nucleic acids and their components are essential, especially in considering the increasing clinical use of ultrasound. Although we intend to continue to conduct our systemic investigation in a dual fashion: 1) sonochemical studies of nucleic acid bases, nucleosides, and nucleotides and polynucleotides at low levels of sonication (less than 5W/cm to the second power, less than 30 min, 1 MHz) and 2) sonobiological studies to establish threshold conditions for inactivation of transforming DNA and for single- and double-stranded breakage of various DNAs, we had until now to focus our attention on the chemical aspects. During the past year we have conducted a systematic study of the effect of variation of ultrasonic parameters including intensity (I(SA)TA and I(SA)TP), pulse width, and duty cycle, in order to compare extents of sonoreaction in CW and pulse modes. Extents of reaction were determined by comparison of heights of the peak corresponding to starting pyrimidine on HPLC analysis or of optical densities at approximately 265 nm in a UV spectrophotometer. Variation of pulse width showed little effect except in the range 10-50 ms where extents of sonoreaction were marked greater with a maximum at approximately 10 ms. Variation of duty cycle at a constant pulse width (10 ms) showed a maximum at 30%. Expriments using radioisotopes were also undertaken and sonoproucts were subjected to 2-dimensional tlc followed by autoradiography and quantitative analysis of the autoradiographs by liquid scintillation counting. Both CW and pulse modes showed no qualitative or quantitative differences. During the next year, we intend to continue analyses and identification of the major sonoproducts of pyrimidines by HPLC and 2-dimensional tlc-autoradiography.